1. Field of the Invention
The present invention relates to a wire bonding method and apparatus and more particularly to a wire bonding method and apparatus that includes a means for correcting the positional offset between a workpiece imaging camera and a capillary.
2. Prior Art
As shown in FIG. 4, in a workpiece 3 which comprises a semiconductor chip 2 installed on a lead frame 1, wires 4 are used for connecting the pads P.sub.1, P.sub.2 . . . of a semiconductor chip 2 and the leads L.sub.1, L.sub.2 . . . of a lead frame 1. The connection between the pads and the leads are done by a wire bonding apparatus as shown, for example, in FIG. 5.
Generally, in wire bonding between the pads and leads, any positional shift or discrepancy between them from predetermined positions is first detected at at least two points on the semiconductor chip 2 and at least two points on the lead frame 1 by a camera 11, and the bonding coordinates for the pads and leads stored beforehand in the bonding apparatus are corrected based upon the detected result.
When the detection of the positional shift is performed by the camera 11, an X-axis motor 12 and Y-axis motor 13 are first driven so that the central axis 11a of the camera 11 is moved directly above each measuring point. After the bonding coordinates are corrected as described above, the capillary 15, through which a bonding wire 4 passes, is moved in the X and Y directions, or horizontally, and in the Z direction, or vertically, and then the wire 4 is bonded between the pads P.sub.1, P.sub.2 . . . of the semiconductor chip 2 and the leads L.sub.1, L.sub.2 . . . of the lead frame 1.
In the process described above, the central axis 11a of the camera 11 and the central axis 15a of the capillary 15 are positionally offset by a distance W. Accordingly, after the positional shifts of points to be bonded have been detected by the camera 11 and the bonding coordinates have been corrected, an XY table or bonding table 16 that has the capillary 15 is moved horizontally the offset distance W by the X-axis motor 12 and Y-axis motor 13, thus bringing the capillary 15 above a first bonding point. Then, the wire 4 is bonded at the points of corrected bonding coordinates by moving the XY table in the X, Y and Z directions by the X-, Y- and Z-axis motors by raising and lowering the capillary arm 17 (or causing the capillary arm 17 to pivot).
In FIG. 5, the capillary arm 17 is pivotally mounted to a bonding head 10a, and the camera 11 is fixed to the bonding head 10a via a camera-holding arm. In this Figure, Xw represents the X-axis component of the offset distance W, and Yw represents the Y-axis component of the offset distance W. The devices disclosed in Japanese Patent Application Laid-Open (Kokai) Nos. 4-317342 and 4-320350 are examples of wire bonding apparatuses of the type described above. The methods disclosed in Japanese Patent Application Laid-Open (Kokai) No. 51-78174 and Japanese Patent Application Publication (Kokoku) No. 57-50059 are examples of methods for correcting the positions of workpieces.
As seen from the above, there is a mechanically determined fixed offset distance W between the central axis 11a of the camera 11 and the central axis 15a of the capillary 15. Accordingly, bonding can only be accurately performed at bonding points by detecting via the camera 11 the amount of positional shift of each workpiece 3, correcting the bonding coordinates, and then moving the capillary 15 (in accordance with a predetermined program) a distance that corresponds to the offset distance so that the capillary 15 is at the corrected bonding coordinates.
However, a wire bonding apparatus includes a heating block for heating lead frames, and it also includes a capillary arm which holds the capillary. In addition to the heating block, other heat-generating sources such as X- and Y-axis motors and an ultrasonic oscillation source which is installed inside the capillary arm are used in a wire bonding apparatus. A Z-axis motor which raises and lowers or pivots a capillary arm is another element used in the bonding apparatus.
Due to the operating heat and variations in the ambient temperature caused by the heat-generating sources, differences are created between the thermal expansion of the capillary arm and the thermal expansion of the camera-holding arm that holds the camera. As a result, the offset distance between the central axis of the camera and the central axis of the capillary tends to change in terms of both amount and direction, and error caused by this fluctuation results in a shift in the bonding position.
Presently, the correction of such fluctuations in the offset is performed manually by the operator on a periodic basis.
Such correction is done, for instance, in the following manner:
(1) After bonding has been performed between the pad P.sub.1 of the semiconductor chip 2 and the lead L.sub.1 of the lead frame 1 as seen in FIG. 4, the XY table 16 shown in FIG. 5 is moved by an amount that is equal to the offset distance W which is added to the coordinates for the pad P.sub.1, so that the central axis 11a of the camera 11 is moved to the position above the pad P.sub.1 ; PA1 (2) A check is made to see if a ball bonded to the pad P.sub.1 that is imaged by the camera 11 is shifted with respect to the cross-hairs located at the center of a monitor screen; PA1 (3) The XY table 16 is driven manually by operating a ten-key or chessman, so that a ball formed by the bonded wire is aligned with the cross-hairs at the center of the monitor screen; and PA1 (4) Correction is done by adding the amount of movement of the XY table 16 required in this alignment operation to the preset offset distance W. PA1 an offset-correction control memory which stores offset correction time and steps that move the central axis of workpiece detection of the camera to a position above a bonded ball or capillary pressure mark after bonding has been executed, PA1 an offset memory which stores the offset distance, and PA1 an operation controller which drives the XY table via the information stored in the offset-correction control memory so that the central axis of workpiece detection of the camera is moved to a position above the ball or capillary pressure mark and then corrects the offset amount stored in the offset memory in accordance with the amount of shift calculated by the image controller.
The offset correction as described above involves manual operation which needs the bonding apparatus to be temporarily stopped, and then the ball is aligned with the cross-hairs at the center of the monitor screen by operating a manual input means such as a ten-key. Thus, considerable time is required. Furthermore, correction errors are likely to occur as a result of mistakes made by the operator, differences between individual operators, and by other factors. Moreover, in the manual correction, the resolution of the object image is determined by the resolution of the television monitor; and this results in that finer bonding positions cannot be judged, since the positional precision is in pixel (picture element) units. In addition, correction of fluctuations occurring over time is extremely difficult.